1. Field of the Invention
The present invention relates to a solid-state imaging apparatus and an imaging system.
2. Description of the Related Art
In an APS (active pixel sensor) type solid-state imaging apparatus, an amplifying unit such as a source follower amplifier or the like is provided in each of pixels. Here, a 1/f noise or an RTS (random telegraph signal) noise which arises in a MOS (metal oxide semiconductor) transistor used for the amplifying unit causes noises in the amplifying unit. Here, in Japanese Patent Application Laid-Open No. 2005-286168 discloses, it is effective in reducing such noises that a buried type MOS transistor constitutes the source follower amplifier.
Incidentally, Japanese Patent Application Laid-Open No. 2000-244818 discloses that a source follower amplifier is constituted by a depletion type amplifying MOS transistor, and further discloses that the drain voltage of a reset MOS transistor of which one end is connected to a photoelectric converting unit is set to be lower than the drain voltage of the amplifying MOS transistor. It is said that the reason why such a voltage relationship is established is to have a MOS transistor which is used as a constant current load of a signal line, and operate this transistor in a saturation region.
In order to reduce the 1/f noise or the RTS noise, it is effective to form the channel of the amplifying MOS transistor, which constitutes the source follower amplifier, at a position which is deep from the surface of the semiconductor. However, if the channel is formed at the deep position, a threshold voltage decreases (in a case of an N-channel MOS transistor). As a result, there is a case where the amplifying MOS transistor which constitutes the source follower amplifier operates in a region other than a saturation region. Alternatively, there is a case where the amplifying MOS transistor operates in a transition region between the saturation region and a linear region. Here, if the MOS transistor operates in the linear region, linearity of an output signal from the source follower amplifier deteriorates. In particular, in a case where the amplifying MOS transistor is arranged for each pixel or for plural pixels, there is a possibility that the amplifying MOS transistor which operates in the saturation region and the MOS transistor which operates in the linear region coexist due to a variation of these devices.
In consideration of such a problem as described above, the present invention aims to provide a solid-state imaging apparatus which can secure excellent and satisfactory linearity and also can reduce noises.